dvanced ow power techniques sim3l1xx devices · 2020-02-10 · 4.3. code optimization there are...

Rev. 0.1 9/12 Copyright © 2012 by Silicon Laboratories AN725

AN725

ADVANCED LOW POWER TECHNIQUES FOR SiM3L1XX DEVICES

1. Introduction

The system power budget of a low power system has two components: active and low power mode. Active modeperiods include an active core executing code or a larger number of active peripherals. In low power mode periods,the core enters a sleep state and fewer peripherals are active.

Figure 1. Defining the Power Budget—Active and Low Power Modes

SiM3L1xx devices have several features to address reducing power consumption in all operational modes toachieve a longer product lifetime in battery-operated systems. This document addresses each of these featuresand provides guidelines for achieving low power consumption in a variety of configurations and applications.

2. Key Points

This key topics of this document are as follows:

How to reduce active mode time and power consumption

How to reduce low power mode power consumption

Measuring the low power modes on an SiM3L1xx MCU card

General power-saving tips

3. Relevant Documentation

Precision32™ Application Notes are listed on the following website: www.silabs.com/32bit-appnotes.

AN666: Usage Guide for e SiM3U1xx,SiM3C1xx, and SiM3L1xx DMA and DTM Modules

AN720: Precision32™ Optimization Considerations for Code Size and Speed

AN667: Getting Started with the Silicon Labs Precision32 IDE

AN670: Getting Started with the Silicon Labs Precision32 AppBuilder

Active Mode

Low Power Mode

www.silabs.com/32bit-appnotes

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4. Reducing Active Mode Power Consumption

In active mode, the core is fetching instructions from memory and executing those instructions, and a large numberof peripherals may be active at once. This section discusses ways to reduce the SiM3L1xx device powerconsumption in active mode (Normal, PM1, PM4, or PM5).

4.1. Dynamic AHB/APB Clock ScalingOne of the easiest ways to reduce overall system power consumption is to reduce the active mode time, whichmaximizes the amount of time spent in the low power mode.

If the longest path to the next low power mode is the execution of code (e.g., a math algorithm), then it is typicallybeneficial to run the AHB clock at the fastest speed possible to reduce the time spent in active mode.

If the longest path to the next low power mode is the transfer or collection of data through a peripheral, it is best torun the clock at the lowest speed required for the peripheral. For example, if using the UART at 115200 baud andthe core is waiting for data to finish transferring, then running the AHB and APB at the slowest clock to achieve115200 baud may be the lowest power configuration. However, if data is being transferred memory to memory bythe DMA, running the clocks at the fastest speed possible yields the lowest power consumption.

Due to the clock system of the SiM3L1xx devices, the AHB and APB clocks can be dynamically changed quicklyand easily using the CLKCTRL module based on the needs of the application.

4.2. Using the DMA and DTM ModulesThe Direct Memory Access (DMA) and Data Transfer Manager (DTM) modules help move data without coreintervention. This reduces overall power consumption by removing the power consumed during flash accesses.Additionally, since the Cortex-M3 is a load-store architecture where data is loaded into and out of registers only,multiple instructions are required to move data from one area of memory to another, so the DMA and DTM may befaster than data moves by the core, depending on the AHB load.

Instead of performing the data moves, the core can either sleep using wait-for-interrupt (WFI) or wait-for-event(WFE) instructions, or the core can perform other tasks in parallel, reducing the active mode time.

For more information on how to use the DMA and DTM modules, see “AN666:Usage Guide for eSiM3U1xx,SiM3C1xx, and SiM3L1xx DMA and DTM Modules” on the Silicon Labs 32-bit application notes website:www.silabs.com/32bit-appnotes.

4.3. Code OptimizationThere are several different coding techniques, compiler, and library options available for the Precision32 devices.These options will change both code size and execution speed, which may result in power consumption savings insystems which aim to execute code in active mode as quickly as possible before entering a low power state. Inaddition to getting to the low power mode more quickly, changing the project settings may lead to smaller codefootprints, compacting the code into a smaller area with fewer memory accesses. The power consumption benefitsof code optimization for speed or size will vary depending on the project requirements and code.

For more information on how to write efficient code and the various optimization settings, see”AN720:Precision32™ Optimization Considerations for Code Size and Speed” on the Silicon Labs 32-bit application noteswebsite: www.silabs.com/32bit-appnotes.




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4.4. Code DependencyIn addition to dynamic clock management, the power consumption of the SiM3L1xx device will vary with the type ofcode the core executes. For example, if the core executes a complex math routine with branches, the pipeline willmiss every time a branch is taken and new instructions must be fetched. This stall and fetch period causes moreflash accesses, which increases power consumption. In addition, the core executes a wide variety of instructionsand activates the memory bus to fetch data from RAM or flash for use in these routines. In contrast, a string of NOPinstructions will take less power because the core isn’t executing complex instructions.

The data shown in Figure 2 does not use adaptive voltage scaling or any other techniques to change powerconsumption. All of these measurements were taken using the PLL as the clock source with a higher SPMD setting(i.e., reduced flash access frequency) at AHB frequencies above 40 MHz. The APB clock is equal to the AHBclock, when the APB is enabled.

As shown by the data, the flash access frequency (SPMD) has a direct effect on the code that includes branches,since the core stalls when waiting for the new instructions, resulting in reduced power consumption. This meansthat it may be more efficient to run at a faster frequency with the same current consumption to reduce overall timespent in active mode. For the code that is a long string of NOPs, the core never has a pipeline miss and neverstalls, so there is no change in power consumption with a different flash speed mode.

For power sensitive applications, experimenting with various code styles and instruction mixes may result inreduced power consumption in active mode.

Figure 2. Power Consumption Code Dependency

AHB and APB Clock (MHz)

Cu

rren

t

20 25 30 40 504535

NOP code, no APB clocks enabled, all LDOs at 1.8 VNOP code, all APB clocks enabled, all LDOs at 1.8 V

complex code, no APB clocks enabled, all LDOs at 1.8 V

complex code, all APB clocks enabled, all LDOs at 1.8 V

SPMD = 1 SPMD = 2

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4.5. Adaptive Voltage ScalingSiM3L1xx devices have scalable LDOs powering the digital and memory modules on the device. These LDOoutputs are factory calibrated to 1.8 V to handle all process and temperature variations, but this voltage is oftenmuch higher than the minimum voltage required by the digital and memory circuits. During normal operation, theseLDO outputs can be set to a value less than 1.8 V to reduce the amount of excess power consumed by thesecircuits with some buffer to ensure correct operation.

When the digital or memory LDOs are sourced from the dc-dc converter, adaptive voltage scaling can also allowthe LDOs to track the dc-dc output voltage to utilize the higher efficiency of the dc-dc converter and reduce thelosses in the regulators.

Figure 3. Reducing Power Consumption with Adaptive Voltage Scaling

AHB Clock (MHz)

Po

we

r (m

W)

0

10

20

30

40

50

70

60

0 10 20 30 40 50

Digital LDO output fixed at 1.8 V

Digital LDO output scaled once at production test

Digital LDO output scaled adaptively by firmware

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4.6. DC-DC Load and Power EfficiencyThe DCDC0 module on SiM3L1xx devices is a dc-dc buck converter with an input range of 1.8 to 3.8 V and anoutput range of 1.25 to 3.8 V. The efficiency of this regulator changes based on load and the converterconfiguration, as shown in Figure 4. The converter configuration can be changed dynamically or bypassedaccording to the anticipated demands of the application to maintain the highest power efficiency and reduce powerconsumption.

Figure 4. Reducing Power Consumption with the DC-DC Buck Converter

There are three ranges of operation for the dc-dc converter corresponding to three different load sizes:

1. Loads less than 5 mA

2. Loads between 5 and 15 mA

3. Loads greater than 15 mA

When operating with loads less than 5 mA, the dc-dc converter is most efficient when configured for light loads:

1. Power switch size set to 0 (PSMD = 0)

2. Asynchronous mode enabled (ASYNCEN = 1)

3. Minimum pulse width set to 40 ns (MINPWSEL = 3)

For loads between 5 and 15 mA, the converter should be set in a middle configuration:


2. Synchronous mode enabled (ASYNCEN = 0)

3. Minimum pulse width disabled (MINPWSEL = 0)

Current (mA)

Po

wer

Eff

icie

ncy

(%

)

50%

55%

60%

65%

70%

75%

85%

80%

0 10 20 30 40 50

90%

10090807060

Configured for Low Loads

Configured for High Loads

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To configure the converter for loads greater than 15 mA:


2. Synchronous mode enabled (ASYNCEN = 0)

3. Minimum pulse width disabled (MINPWSEL = 0)

Table 1 shows a summary of these settings for each load configuration.

After configuring the dc-dc converter for the load size, switch the appropriate LDOs to the converter output toreduce system power. Each of the three LDOs (memory, digital, and analog) can be switched to the battery voltage(VBAT) or the dc-dc converter output independently.

The firmware can adjust the dc-dc converter configuration based on the anticipated load for the active time. Forexample, if only a few peripherals will be active and the core will halt for a period of time, then the load may be lessthan 5 mA. Firmware can adjust the dc-dc converter appropriately during this period. If the core is fetchinginstructions from flash at full speed and executing a math routine, then switching to the high load configuration willyield lower power consumption.

For extremely light loads (less than 2-3 mA), the dc-dc converter will be less efficient than the LDOs at ~50%efficiency, depending on the VBAT voltage and LDO bias settings. When this occurs, the dc-dc converter should bebypassed (BEN = 1), which connects VBATDC to VDC, or disabled (DCDCEN = 0).

Table 1. DC-DC Load Configurations

LoadPower Switch Size

(PSMD)

Synchronous or Asynchronous

(ASYNCEN)

Minimum Pulse Width (MINPWSEL)

less than 5 mA 0 Asynchronous (1) 40 ns (3)

5 mA to 15 mA 0 Synchronous (0) disabled (0)

greater than 15 mA 3 Synchronous (0) disabled (0)

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5. Reducing Power Consumption in Low Power Modes

In low power mode, the device core halts and the number of active peripherals typically reduces to the minimumrequired by the application (i.e., real time clock). This section discusses techniques to reduce the SiM3L1xx devicepower consumption in low power modes (PM2, PM3, PM6, and PM8).

5.1. SiM3L1xx Low Power Mode OverviewThe SiM3L1xx devices feature seven low power modes in addition to normal operating mode. Several peripheralsprovide wake up sources for these low power modes, including the Low Power Timer (LPTIMER0), RTC0 (alarmsand oscillator failure notification), Comparator 0 (CMP0), Advanced Capture Counter (ACCTR0), LCD VBATmonitor (LCD0), UART0, low power mode charge pump failure, and PMU Pin Wake.

In addition, all peripherals can have their clocks disabled to reduce power consumption with the clock control(CLKCTRL) registers whenever a peripheral is not being used.

The SiM3L1xx devices have the power modes defined in Table 2.

5.1.1. Normal Mode (Power Mode 0)

Normal mode encompasses the typical full-speed operation. The power consumption of the device in this mode willvary depending on AHB/APB clock speeds, the settings of CLKCTRL and the peripherals, and the dc-dc converterand LDO settings.

5.1.2. Power Mode 1

Power Mode 1 occurs when the core executes code from RAM instead of flash. The power consumption of thedevice is less than normal mode when in PM1.

5.1.3. Power Mode 2

In Power Mode 2, the core halts and the peripherals run at full speed. To place the device in this mode, the clocksettings in CLKCTRL should remain the same as Normal or Power Mode 1 and the core should execute a WFI orWFE instruction. If the WFI instruction is called from an interrupt service routine, the interrupt that wakes the devicefrom PM2 must be of a sufficient priority to be recognized by the core. It is recommended to perform both a DSB(Data Synchronization Barrier) and an ISB (Instruction Synchronization Barrier) operation prior to the WFI toensure all bus access is complete.

5.1.4. Power Mode 3 Fast Wake

Power Mode 3 Fast Wake occurs when all the clocks are stopped except for the LFOSC0 or RTC0TCLK. The coreand the peripherals are halted in this mode. The available wake up sources to wake from PM3 are controlled by thePower Management Unit (PMU). The available wake up sources are: Low Power Timer (LPTIMER0), RTC0(alarms and oscillator failure notification), Comparator 0 (CMP0), advanced capture counter (ACCTR0), LCD VBATmonitor (LCD0), UART0, and PMU Pin Wake. Any reset event will also wake the device from PM3.

If the WFI instruction that wakes the device from PM3 Fast Wake is called from an interrupt service routine, theinterrupt that wakes the device from PM3FW must be of a sufficient priority to be recognized by the core.

By keeping the core clock running at a slow frequency in PM3 and changing the AHB and APB clocks to the LowPower Oscillator, the device can wake up faster than in standard Power Mode 3 at the expense of higher powerconsumption.

5.1.5. Power Mode 3

Power Mode 3 occurs when all the clocks are stopped, and the core and the peripherals are halted. Waking fromPM3 requires one of the PMU wake sources described in “5.1.4. Power Mode 3 Fast Wake” to be properlyconfigured.

If the WFI instruction is called from an interrupt service routine, the interrupt that wakes the device from PM3 mustbe of a sufficient priority to be recognized by the core.

5.1.6. Power Mode 4

Power Mode 4 is the same as normal operation except the AHB clock operates at a slower speed. The powerconsumption of the device in this mode will vary depending on the AHB/APB clock speeds, the settings ofCLKCTRL and the peripherals, and the dc-dc converter and LDO settings.

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5.1.7. Power Mode 5

Power Mode 5 is the same as PM1 with a slower AHB clock source selected. The core executes code from RAMinstead of flash. The power consumption of the device in PM5 is slightly less than PM4.

5.1.8. Power Mode 6

In Power Mode 6, the core halts and the peripherals run at a slower speed than PM2. To place the device in thismode, the clock settings in CLKCTRL should remain the same as PM4 or PM5, and the core should execute a WFIor WFE instruction. If the WFI instruction is called from an interrupt service routine, the interrupt that wakes thedevice from PM6 must be of a sufficient priority to be recognized by the core.

5.1.9. Power Mode 8

In Power Mode 8, the core and most peripherals are halted, most clocks are stopped, and registers retain theirstate. In addition, the LDO regulators are disabled, so all active circuitry operates directly from VBAT. Alternatively,the PMU has a specialized VBAT-divided-by-2 low power mode charge pump that can power some internalmodules while in PM8 to save power. The fully operational functions in this mode are: LPTIMER0, RTC0, UART0running from RTC0TCLK, port match, advanced capture counter, and the LCD controller.

This mode provides the lowest power consumption for the device, but requires an appropriate wake up source orreset to exit. The available wake up or reset sources to wake from PM8 are controlled by the Power ManagementUnit (PMU). The available wake up sources are: Low Power Timer (LPTIMER0), RTC0 (alarms and oscillatorfailure notification), Comparator 0 (CMP0), advanced capture counter (ACCTR0), LCD VBAT monitor (LCD0),UART0, low power mode charge pump failure, and PMU Pin Wake. The available reset sources are: RESET pin,VBAT supply monitor, Comparator 0, Comparator 1, low power mode charge pump failure, RTC0 oscillator failure,or PMU wake event.

To enter this mode, firmware must write the SLEEPDEEP bit in the ARM System Control Register. Firmware mustthen execute a WFI or WFE instruction. The core will remain in PM8 until an enabled wake up or reset sourceoccurs.

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Table 2. SiM3L1xx Power Modes

Mode Description Mode Entrance Mode Exit

Normal Core operating at full speed

Code executing from flash

Power Mode 1 (PM1)

Core operating at full speed

Code executing from RAMExecute code from RAM Jump to code in flash

Power Mode 2 (PM2)

Core halted

AHB and APB operate at full speed for peripherals

WFI or WFE instruction NVIC or WIC wakeup

Power Mode 3 Fast Wake (PM3FW)

All clocks stopped except LFOSC0 or RTC0TCLK

AHB and APB set to Low Power Oscillator

Core clock set to LFOSC0 or RTC0TCLK

DMACTRL0 disabled

Fast wake mode enabled in PM3CN

AHB switched to Low Power Oscillator

WFI or WFE instruction

Requires a wake up source or reset defined by

the PMU

Power Mode 3 (PM3)

All clocks stopped

DMACTRL0 disabled

Clocks disabled in PM3CN

WFI or WFE instruction

Requires a wake up source or reset defined by

the PMU

Power Mode 4 (PM4)

Core operating at slower speed

Code executing from flash

Set the AHB clock to a slower source

Set the AHB clock to a faster source

Power Mode 5 (PM5)

Core operating at slower speed

Code executing from RAM


Execute code from RAM

Set the AHB clock to a faster source or jump to

code in flash

Power Mode 6 (PM6)

Core halted

AHB and APB operate at a slower speed for peripherals


WFI or WFE instructionNVIC or WIC wakeup

Power Mode 8 (PM8)

Low power sleep

The LDO regulators are disabled and all active circuitry operates directly from VBAT

The following functions are available: ACCTR0, RTC0, UART0 running from RTC0TCLK, LPTIMER0, port match, and the LCD controller

Register state retention

SLEEPDEEP set in the ARM System Control Register

WFI or WFE instructionRequires a wake up

source or reset defined by the PMU

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5.2. Measuring PowerThis section discusses the hardware setup and code required to reproduce the power specifications reported bythe SiM3L1xx data sheet.

5.2.1. Hardware Setup

To measure the power on the SiM3L1xx MCU Card with a fixed 3.3 V VBAT:

1. Connect a USB Debug Adapter to the 10-pin CoreSight connector (J14).

2. Remove the three shorting blocks from J7.

3. Remove the JP2 Imeasure shorting block and put a multimeter across (positive side on the bottom pin).

4. Move the VBAT Sel switch (SW2) to the middle +3.3 V_VREG position.

5. Move the VIO Sel (SW8) and VIORF Sel (SW9) switches to the bottom VBAT position.

6. Connect the 9 V power adapter to POWER (J6).

7. Download the code to the board.

8. Remove the debug adapter connection.

9. Measure the power of the device.

Figure 5. SiM3L1xx MCU Card Power Measurement Configuration—Fixed VBAT

USB Debug Adapter

1

2

3

6

9 V Power Adapter

4

5

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To measure the power on an SiM3L1xx MCU Card with a varying VBAT:

1. Connect a USB Debug Adapter to the 10-pin CoreSight connector (J14).

2. Remove the three shorting blocks from J7.

3. Remove the JP2 Imeasure shorting block.

4. Move the VBAT Sel switch (SW2) to the bottom BAT position.

5. Move the VIO Sel (SW8) and VIORF Sel (SW9) switches to the bottom VBAT position.

6. (Optional) Place a battery in the BT1 holder.

7. (Optional) Instead of a battery, connect the positive terminal of a bench power supply to the positive terminal of the multimeter, the negative terminal of the power supply to the MCU card ground, and the negative terminal of the multimeter to the top pin of the Imeasure jumper (JP2). This configuration will prevent any circuitry on the board from interfering with the power measurement.

8. Download the code to the board.

9. Remove the debug adapter connection.

10. Measure the power of the device.

Figure 6. SiM3L1xx MCU Card Power Measurement Configuration—Varying VBAT

+000.0075 µADC

2.8 V

USB Debug Adapter

1

2

3

6

4

5

9

7

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5.2.2. Configuring a Device for Normal (PM0) and Power Mode 4

In Normal (PM0) and Power Mode 4, the device executes code from flash. The only difference between PM0 andPM4 is the core clock speed (fast and slow, respectively).

To configure a device to run in PM0 or PM4:

1. Enable the clocks to peripherals that will be configured by firmware.

2. Select the desired clock source (LPOSC0, PLL0, etc.) and speed for both AHB and APB clocks.

3. Select the desired adaptive voltage scaling settings using the LDO module.

4. (Optional) Disable the retention mode of any enabled RAM banks.

5. Set the pins in the lowest power configuration for this mode.

6. Disable all unused peripherals.

7. Disable the clocks to all unused peripherals.

8. Jump to code in flash.

To measure the data sheet numbers using an SiM3L1xx MCU Card and the AN725_PowerModes_0_and_4example:

1. Configure the SiM3L1xx MCU Card according to the instructions in “5.2.1. Hardware Setup”.

2. Open the AN725_PowerModes_0_and_4 example in either Keil µVision or the Precision32 IDE.

3. Select the desired settings using the #defines at the top of the file. There is a set of defines for each data sheet specification.

4. Compile and download the code to the device.

5. Disconnect the USB Debug Adapter.

6. Reset the device.

7. Measure the power consumption of the device.

5.2.3. Configuring a Device for Power Modes 1 and 5

In Power Modes 1 and 5, the device executes code from RAM rather than flash. The only difference between PM1and PM5 is the core clock speed (fast and slow, respectively).

Keil µVision uses a scatterfile with a retention RAM tag defined as MCU_RAM_CODE:

MCU_IRAM +0 {

.ANY (+RW +ZI)

.ANY (MCU_RAM_CODE)

}

The project source file can then define a tag in code to place functions in this section of memory:

#define __SI32_RRAM __attribute__ ((section("MCU_RAM_CODE")))

Any functions that use the __SI32_RRAM tag in the definition will be placed in RAM starting at 0x2000_0000:

__SI32_RRAM void run_PM1(void)

{

...

}

For Precision32 projects, a function can be located in RAM by treating it like normal data and applying a data

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section attribute tag to it.

__attribute__ ((__section__(".data.name")))

This tag will result in a warning indicating that code is being placed in the data section, but this warning can beignored. As an example, placing the PowerModes_2_or_6() function in RAM would look like:

__attribute__ ((__section__(".data.pm2_or_6")))

void PowerModes_2_or_6(void)

{

...

}

There are two ways to access the function in RAM: direct call or function pointer. Direct calls can cause someissues with debugging the code in RAM, and the debugger may just step over the function instead of stepping intoit. To work around this, single step at the instruction level, set a breakpoint in the RAM code, or use a functionpointer.









8. Jump to code in RAM.

9. (Optional) Disable the AHB clock to the flash controller.




3. Select the desired settings using the #defines at the top of the file. There is a set of defines for each data sheet specification.





The code executes from RAM in this mode, so all functions called should also be in RAM.

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5.2.4. Configuring a Device for Power Modes 2 and 6

For Power Modes 2 and 6, the core halts and no longer executes code, but the active peripherals still run from theAPB clock. The only difference between PM2 and PM6 is the APB clock speed (fast and slow, respectively).





4. Disable Power Mode 8 in the Clock Control module.



7. Set up the wakeup interrupt source (including priority).

8. Disable the SysTick timer to prevent these interrupts from waking the core.



11. Jump to code in RAM.

12. Execute the DSB (Data Synchronization Barrier), ISB (Instruction Synchronization Barrier), and WFI (Wait for Interrupt) or WFE (Wait for Event) instructions. For SiM3L1xx devices, WFI and WFE have the same behavior.




3. Select the desired AHB clock rate using the #defines at the top of the file.




7. Press PB1.7 to place the device in PM2 or PM6. The PB1.5 LED will turn off.


9. Press PB1.5 to wake the device from PM2 or PM6. When this happens, the PB1.7 LED will turn on and the core will sit in an infinite while(1) loop.

The interrupt that wakes the core from the halted state must have sufficient priority if the WFI or WFE instructionwas called from within an ISR.

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5.2.5. Configuring a Device for Power Mode 3 Fast Wake

In PM3 Fast Wake, all clocks are stopped except for a very low frequency clock (RTC0OSC or LFOSC0) thatallows the core to wake up faster than standard PM3.

To configure a device to run in PM3 Fast Wake:



3. Disable Power Mode 8 mode in the Clock Control module.

4. Set the AHB clock to the low power oscillator (LPOSC0).

5. Select the fast wake clock source using the PM3CSEL field in the CLKCTRL PM3CN register.

6. Enable fast wake mode by setting the PM3CEN bit to 1 in the PM3CN register.



9. Set up the desired PMU wakeup source.

10. Put all of the LDOs in low bias mode.

11. Clear the PMU wakeup flags.

12. Disable the SysTick timer.



15. Execute the DSB, ISB, and WFI or WFE instructions. For SiM3L1xx devices, WFI and WFE have the same behavior.

To measure the data sheet numbers using an SiM3L1xx MCU Card and the AN725_PowerMode3_Fast_Wakeexample:


2. Open the AN725_PowerMode3_Fast_Wake example in either Keil µVision or the Precision32 IDE.




6. Connect PB0.2 to PB1.5 using a short wire and the J21 and J22 headers.


8. Press PB1.7 to place the device in PM3FW. The PB1.5 LED will turn off.


10. Press PB1.5 to wake the device from PM3FW. When this happens, the PB1.7 LED will turn on and the core will sit in an infinite while(1) loop.

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5.2.6. Configuring a Device for Power Mode 8

PM8 is the lowest power mode where all clocks are stopped and only the PM8 peripherals are available. The RAMbanks do not have retention enabled by default, so firmware must enable each bank to retain data through PM8.For most applications, it is recommended to enable retention for all RAM banks, but the specific bank containingthe stack pointer must be enabled to ensure proper code operation on an exit from PM8. See “5.5. Retention RAM”for more information on changing the location of the stack pointer.

To configure a device to run in PM8:


2. Enable PM8 mode by setting the PMSEL bit to 1 in the CLKCTRL CONFIG register.

3. Set up the desired PMU wakeup source.

4. Switch the AHB clock to the 20 MHz Low Power Oscillator and adjust the flash read timing as necessary.

5. Enable the retention mode of RAM banks needed by the application (all eight banks recommended for most applications).

Note: To properly execute code after exiting PM8, the stack pointer must be located in a RAM bank with retention enabled.


7. Set all LDOs to 1.5 V for VBAT between 1.8 to 2.9 V and to 1.9 V for VBAT between 2.0 to 3.8 V output.

8. (Optional) If VBAT is between 2.0 to 3.8 V, set the VBAT Monitor threshold to the high setting.

9. Set up the low power peripherals as desired (RTC0, ACCTR0, LCD0, etc.).

10. (Optional) If using the RTC in low-frequency oscillator mode, enable the RTC0 module using the steps in the Reference Manual. If using the RTC in crystal mode, the recommended initialization sequence is:

a. Disable RTC0 automatic gain control (AGC) and enable the bias doubler.

b. Set up the RTC0 in crystal mode using the list of steps in the Reference Manual.

c. (Optional) Enable the charge pump (see Step 10).

d. Enable RTC0 automatic gain control and disable the bias doubler.

11. (Optional) Configure and enable the charge pump:

a. Enable the low power charge pump monitor in the PMU.

b. Enable the RTC0 clock to other modules (CLKOEN = 1 in the RTC0 module).

c. Enable the low power charge pump monitor as a reset source.

d. Set the low power charge pump load to the appropriate value for the application. See “5.3. Low Power Mode Charge Pump” for more information.

e. Enable the low power charge pump and enable it as a PMU wake up source.

f. Enable the low power charge pump interrupt.

12. Disable all undesired reset sources.

Note: The Watchdog Timer (WDTIMER0) and PVTOSC0 modules will behave as if a power-on reset occurred after exitingPM8, so disabling the Watchdog Timer as a reset source will prevent unwanted resets.

13. Disable the SysTick timer.


15. (Optional) Disable the clocks to all unused peripherals. Entering PM8 stops all clocks, so this step is not necessary for this mode.

16. Clear the PMU wakeup flags.

17. Enable the interrupts for the PMU wakeup source (enabled in Step 3).

18. Set the SLEEPDEEP bit in the core.

19. Execute the DSB, ISB, and WFI or WFE instructions. For SiM3L1xx devices, WFI and WFE have the same behavior.

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To measure the data sheet numbers using an SiM3L1xx MCU Card and the AN725_PowerMode_8 example:

1. Configure the SiM3L1xx MCU Card according to the instructions in Section “5.2.1. Hardware Setup”.

2. Open the AN725_PowerMode_8 example in either Keil µVision or the Precision32 IDE.




6. Connect PB0.2 to PB1.5 using a short wire and the J21 and J22 headers.

7. Power down the board.

8. Power up the board. This will cause a power-on reset that will clear the settings of some modules (i.e., PMU).

9. Press PB1.7 to place the device in PM8. The PB1.5 LED will turn off.


11. Press PB1.5 to wake the device from PM8. When this happens, the PB1.7 LED will turn on and the core will sit in an infinite while(1) loop.

AN725

18 Rev. 0.1

5.3. Low Power Mode Charge PumpThe SiM3L1xx devices feature a low-power, voltage-halving charge pump used to power most of the functionsoperating in the lowest power mode (PM8). This charge pump always powers the low-frequency and 32 kHz crystaloscillators. When in PM8, this charge pump also powers the following digital components: RTC counters andalarms, LPTIMER0, UART0, ACCTR0, retention RAM banks, LCD0, and the Power Management Unit (PMU) andreset controllers. When not in PM8, these modules are powered by the Memory regulator (LDO0). The analogcircuitry of the LCD and Pulse Counter is always powered directly from the VBAT pin.

The low power mode charge pump is a voltage-halving circuit which reduces the sleep-mode supply current usingtwo methods:

1. It provides approximately VBAT divided-by-2 to the oscillators and digital logic, reducing their inherent power consumption.

2. It provides a transformer effect which causes the current pulled from the VBAT input to be ideally one-half of the current actually consumed by the circuitry powered by the charge pump.

This charge pump can be modeled as a VBAT divided-by-2 supply with a series impedance of RCPLOAD.Equation 1 shows the equation for the output voltage of the charge pump.

Equation 1. Charge Pump Output Voltage

The value of RCPLOAD depends on the output drive setting field (CPLOAD) in the PMU module and the clockselected by the RTC0 module, as shown in Table 3.

All circuitry powered by the charge pump operates correctly at VCP voltages greater than or equal to 0.95 V. Thus,for a given charge pump load current, Equation 2 must be true.

Equation 2. Charge Pump Output Voltage Requirement

In all applications with a VBAT voltage of 2.4 V or above, the charge pump can operate at its minimum driveimpedance (CPLOAD = 3). However, as shown in Table 3, the overhead current consumed by the charge pumpincreases as its drive impedance decreases. Therefore, at lower values of ILOAD, the total PM8 currentconsumption can be reduced by increasing the charge pump’s drive impedance. In addition, since a higher driveimpedance corresponds to a lower output voltage for a given load current, increasing the drive impedance alsoreduces the current consumed by the load circuitry.

At higher temperatures, using the low power charge pump and RTC0 can help reduce the leakage current of thedevice. Reducing CPLOAD can further reduce the leakage at these temperatures.

Table 3. Charge Pump Output Drive Settings

CPLOAD fieldRCPLOAD Value Overhead Current

LFOSC (16.4 kHz) RTCOSC (32 kHz) LFOSC (16.4 kHz) RTCOSC (32 kHz)0 ~600 k ~300 k ~0 nA ~0 nA

1 ~200 k ~100 k ~25 nA ~50 nA

2 ~90 k ~45 k ~50 nA ~100 nA

3 ~40 k ~20 k ~75 nA ~150 nA

VCPVBAT

2---------------- ILOAD RCPLOAD–=

VBAT2

---------------- ILOAD RCPLOAD– 0.95 V

AN725

Rev. 0.1 19

5.3.1. Monitor

The SiM3L1xx devices include a low-power voltage monitor that compares the output of the low power modecharge pump against a factory-trimmed 0.95 V reference voltage. This monitor can serve as an interrupt, a PM8wakeup source, and a reset source. It is highly recommended that this monitor be enabled and used as a wakeupor interrupt source. However, if Equation 2 will always be true under all valid operating conditions, the monitor isnot strictly required. Leaving the monitor disabled introduces the risk that the device could lock-up, requiring anexternal reset or power cycle to recover. This can occur if the selected RTC0 oscillator is disturbed and stopsoscillating, thereby causing the charge pump output to fail. Since the charge pump powers the RTC0 oscillators,the RTC fail notification is not sufficient to detect this condition, and it will not correct itself without externalintervention. The charge pump voltage monitor consumes approximately 15 nA, when enabled.

When used as a wakeup and interrupt source, the charge pump monitor can alert the firmware that the chargepump output is nearing 0.95 V. This may occur as temperature rises, for example, causing additional thermalleakage from the PM8 circuitry. The firmware can respond by decreasing the charge pump drive impedance byincreasing CPLOAD, assuming the drive is not yet at its minimum. Otherwise, firmware must either reduce the PM8load (by disabling RAM banks or other PM8 active modules) or place the charge pump in bypass mode. Whenchanging the charge pump mode, it is recommended that all digital logic using the RTC0 oscillators be temporarilydisabled. Otherwise, a glitch from the oscillators due to the sudden change in the charge pump output may disturbthe logic’s operation.

5.3.2. Measuring PM8 Charge Pump Current

To measure the charge pump load current, use CMP0 to detect the output voltage of the charge pump during PM8.This output voltage, combined with an ADC measurement of the VBAT value, can help estimate the present chargepump load in PM8. To do this:

1. Enable the charge pump with CPLOAD set to 3.

2. Configure the charge pump monitor to wake up and interrupt the device.

3. Configure CMP0 to wake and interrupt the device if the charge pump output voltage is equal to 16 x VBAT/64 (0x10) using the comparator DAC feature.

4. Schedule an RTC0 alarm to wake and interrupt the device in 1 ms.

5. Enter PM8 and wait for a wakeup event.

6. If the wakeup event occurred from the voltage monitor, this indicates the present VBAT voltage is too low to operate the charge pump. The charge pump should be configured for bypass mode and this measurement aborted.

7. If the wakeup event was the comparator, change the CMP0 to compare against 8 x VBAT/64 (0x08).

8. If the wakeup event was the RTC0 alarm, change the CMP0 to compare against 24 x VBAT/64 (0x18).

9. Repeat this process starting at step 4, continuing to change the CMP0 DAC using a successive-approximation method to obtain a 5-bit digital value for the charge pump output voltage, where 0x1F corresponds to 31 x VBAT/64.

10. Use the SARADC0 module to measure the voltage on the VBAT input.

11. Calculate the estimated charge pump load current using Equation 1, the VBAT and final CMP0 DAC value, and the appropriate value for RCPLOAD.

If the resulting load current is small enough that the next lowest setting of CPLOAD results in a charge pump outputgreater than 0.95 V, this sequence can be repeated with CPLOAD set to 2 to obtain a more accurate measurementof the load current, and so forth.

AN725

20 Rev. 0.1

5.3.3. Dynamic Charge Pump Drive Impedance

The low power mode charge pump supports four drive impedance settings. For applications requiring aggressivePM8 current reduction, these settings can be adjusted dynamically to minimize the total low power mode currentconsumed. This adjustment could be performed by measuring the charge pump load current as described inSection 5.3.2 before each entry into PM8 and setting the appropriate drive impedance. However, a faster andsimpler method is available.

The charge pump dynamic scaling method consists of two parts:

1. Determine when the drive impedance should be decreased by using the charge pump voltage monitor. Each time the monitor trips, firmware should reduce the drive impedance setting. If the setting is already at its minimum, firmware can place the charge pump in bypass mode or disable active PM8 circuitry.

2. Determine when the drive impedance can be increased. On a regular but relatively infrequent basis, firmware can determine if the CPLOAD setting can be decreased by one without causing the charge pump output to fall below 0.95 V as described in this section.

The output state of the charge pump in PM8 is given by Equation 1. The required state of the charge pump ifCPLOAD is reduced by 1 is given by:

Equation 3. Charge Pump Output Voltage at an Increased Impedance

In Equation 3, the RCPLOAD-1 term is the drive impedance for CPLOAD-1. Putting these equations together andsolving for ILOAD gives:

Equation 4. Minimum Charge Pump Output Voltage to Increase Drive Impedance

Equation 4 provides the minimum VCP value for the present CPLOAD setting that allows CPLOAD to be reducedby 1 without the output dropping below 0.95 V. Using this equation, the CMP0 DAC setting that corresponds to thisequation can be written as:

Equation 5. CMP0 DAC Setting for Minimum Charge Pump Output Voltage to Increase Drive Impedance

On a regular basis, firmware can perform the following steps:

1. Use the SARADC0 module to measure VBAT.

2. Set the CMP0 DAC to the value determined by Equation 5.

3. Configure CMP0 to wake and interrupt the device if the charge pump output voltage is less than the CMP0 DAC.

4. Schedule an RTC0 alarm to wake and interrupt the device in 1 ms.

5. Enter PM8 and wait for a wakeup event.

6. If the wakeup event was the comparator, CPLOAD should not be reduced by 1. Otherwise, CPLOAD can be reduced by 1.

Equation 4 includes some safety margins that might not be immediately obvious, since it assumes that the chargepump load current is independent of the charge pump output voltage. In fact, the PM8 circuitry requires less currentas its supply lowers. Since reducing CPLOAD by 1 will decrease the charge pump output voltage, the resultingoutput voltage should always be greater than 0.95 V, assuming Equation 4 is satisfied.

VBAT2

---------------- ILOAD RCPLOAD 1–– 0.95 V

VCPVBAT

2---------------- 1

RCPLOAD

RCPLOAD 1–--------------------------------–

0.95 VRCPLOAD

RCPLOAD 1–-------------------------------- +

DAC ceiling 32 1RCPLOAD

RCPLOAD 1–--------------------------------–

64 0.95 VVBAT

------------------------------RCPLOAD

RCPLOAD 1–-------------------------------- +

=

AN725

Rev. 0.1 21

5.4. LCDFor applications that use the LCD0 module, the SiM3L1xx devices have several features that help reduce powerconsumption.

5.4.1. Segment Resetting

Each segment of an LCD can be modeled as a capacitor in the order of tens of pF. A typical load current is 1 nA persegment per pF, so a 160-segment LCD with 50 pF per segment can draw 8 µA of load current. The SiM3L1xxdevices include a segment resetting feature to reduce this load current. Figure 7 illustrates the basic architecturemodel of an LCD.

Figure 7. LCD Model

LCD segments are energized (i.e., turn opaque) by ac potential waveforms between the segment and the commonsignal for that segment. These waveforms transition between 0 and 3 V, for example, which results in an amplitudeof –3 to 3 V on the segment, depending on the phase of the segment and common waveforms. Figure 9 shows anexample of the common waveforms for 4-mux mode.

To turn on a segment, the controller drives the segment to 3 V or 0 V when the common is driven to 0 V or 3 V,which leads to ±3 V on the segment. To turn a segment off, the controller drives the segment to 1 V or 2 V when thecommon signal is 0 V or 3 V, which leads to ±1 V on the segment.

Most LCD controllers transition directly from the most positive amplitude (3 V) to the most negative amplitude (–3 V), as shown in Figure 8. This behavior leads to extra charge being pulled from the battery, since the batterymust drive a 6 V transition overall.

Figure 8. Traditional LCD Segment Transition

COM0

COM1

COM2

COM3

SEG0 SEG1 SEG7SEG6SEG5SEG4SEG3SEG2

Segment

+3 V

3 V 3 V

+3 V

3 V 3 V

-3 V

3 V 3 V

Segment

AN725

22 Rev. 0.1

Figure 9. LCD Common Waveforms

For a 4 MUX LCD architecture, Figure 9 demonstrates that three of the four commons are at the same potential inevery phase. This potential is 2 V for the even phases and 1 V for the odd phases. Instead of the traditionalsegment switching method, the SiM3L1xx devices reset the segments to 2 V for even phases and 1 V for oddphases before switching to the next segment potential to reduce the overall load current of the LCD.

The waveforms shown in Figure 10 show the SiM3L1xx segment resetting behavior in the common and segmentwaveforms. The areas marked in red are the reset events to the 1 V or 2 V potential, depending on the phase.

Overall, this reset waveform scheme reduces the current load of an LCD by ~40%, regardless of the number ofsegments in the display. Power-conscious applications using an LCD should enable this feature (RPHEN = 1) andset the number of RTC0 clocks to reset the segment (RPHMD field) using the LCD0 SEGCONTROL register.Generally, a RPHMD value of 2 provides significant power savings without introducing visual artifacts. However, itis recommended to try a range of values to find the best value for a given LCD.

Figure 10. SiM3L1xx LCD Waveforms with Segment Resetting

COM0

COM1

0 1 65432 7

Phase

COM2

COM3

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

COM0

SEG0

0 1 65432 7

Phase

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

0 V

3 V

2 V

1 V

2 V

1 V

2 V

1 V

AN725

Rev. 0.1 23

5.4.2. Contrast Modes

The LCD0 module features four contrast modes: bypass, minimum, constant, and auto-bypass. The selection ofthe contrast mode is governed by the application requirements and LCD specifications. The power consumption ofthese modes will vary due to the internal charge pump and resistor string that generates the VLCD voltage.Reducing the time this hardware is active will reduce power consumption.

In bypass mode shown in Figure 11, the VLCD voltage tracks with the VBAT input voltage. This mode reducespower consumption by disabling the LCD charge pump and is most appropriate for systems where the VBATvoltage is relatively constant.

Figure 11. LCD0 Bypass Contrast Mode

In minimum contrast mode shown in Figure 12, the VLCD voltage will track the VBAT voltage to a minimum levelset by the VBAT monitor threshold (VBMTH). When VBAT is equal to this voltage, VLCD will switch to the contrastlevel set by the CTRST field. The threshold and contrast voltages do not have to be set to the same level, asshown in Figure 13. If the VBAT voltage goes back above the threshold, the LCD contrast charge pump will turn off.This mode is most efficient for systems that can use the battery voltage most of the time but require a minimumcontrast level to see the LCD segments.

Figure 12. LCD0 Minimum Contrast Mode

Figure 13. LCD0 Minimum Contrast Mode with Different Threshold and Voltage Settings

VBATVLCD

VBATVLCD

minimum level set by CTRST

VBATVLCD CTRST

VBMTH

AN725

24 Rev. 0.1

In constant contrast mode, the VLCD voltage is held constant at all times, regardless of the voltage on VBAT. WhenVBAT is above the threshold set by the VBMTH field, the LCD charge pump regulates to the contrast level set bythe CTRST field using a variable resistor. When VBAT reaches the threshold, the charge pump enablesautomatically to keep VLCD at the desired value. This mode results in the highest power consumption, but may berequired by some system and LCD architectures.

Figure 14. LCD0 Constant Contrast Mode

In auto-bypass contrast mode shown in Figure 15, the VLCD voltage is held constant and tracks the VBAT voltagebelow the threshold. When VBAT is above the threshold voltage set by VBMTH, VLCD regulates to theprogrammed contrast voltage (CTRST) using a variable resistor between VBAT and VLCD. When VBAT reachesthe monitor threshold, the controller automatically enters bypass mode and powers VLCD directly from VBAT. Thecharge pump is always disabled in this mode, and the threshold and contrast levels do not have to be set to thesame value, as shown in Figure 16.

Figure 15. LCD0 Auto-Bypass Contrast Mode

Figure 16. LCD0 Auto-Bypass Contrast Mode with Different Threshold and Voltage Settings

For lowest power consumption, applications should use bypass or minimum contrast modes whenever possible.

VBAT

VLCD

VBAT

VLCD

VBAT

VLCDCTRST

VBMTH

AN725

Rev. 0.1 25

5.4.3. Refresh Rate

The LCD0 module on SiM3L1xx devices includes a programmable refresh rate by setting the CLKDIV bits in theCLKCONTROL register. The LCD multiplexor mode must be taken into account when determining the prescalervalue. The LCD power consumption will scale proportionally with the refresh rate. For maximum power savings,choose the slowest LCD refresh rate and the minimum LCD0 clock frequency that provides acceptableperformance for the application. For the least flicker, choose a fast LCD refresh rate.

5.5. Retention RAMThe retention RAM on SiM3L1xx devices is in eight 4 kB banks. All eight banks are available when not in PM8, butcan be individually powered down when in PM8. The current leakage of the RAM at higher temperatures mayincrease, so disabling any unused banks when entering a low power mode may reduce system powerconsumption.

For most applications, it is recommended to enable retention for all RAM banks. The specific bank containing thestack pointer must be enabled to ensure proper code operation on an exit from PM8.

5.5.1. Adjusting the Stack Pointer with Keil µVision

With Keil µVision projects, the stack location can be controlled in the scatterfile. The defaultlinker_sim3l1xx_arm.sct scatterfile can be found in the C:\SiLabs\32bit\si32-x.y.z\si32Hal\sim3l1xx directory,where x is the major si32HAL version, y is the minor version, and z is the trivial version.

Inside the scatterfile:

ARM_LIB_STACK (SI32_MCU_RAM_BASE + SI32_MCU_RAM_SIZE)

The value in the parentheses after the ARM_LIB_STACK label is the location of the stack pointer in memory.Change this value as needed for the application and ensure this RAM bank has retention enabled.

5.5.2. Adjusting the Stack Pointer with the Precision32 IDE

In the Precision32 IDE, projects can use a custom linker script file edited manually or the IDE dialogs to modify thestack pointer address. To add a custom linker script file manually:

Right-click on the project name in the Project Explorer view and select NewFolder. This folder must use thelinkscripts name.

Copy the link_template.ld file from the Precision32 installation directory ..\IDE\precision32\Wizards\linker to theproject’s linkscripts folder.

Open the project’s link_template.ld file and edit the stack line near the bottom of the file:

PROVIDE(_vStackTop = __top_${DATA} - ${STACK_OFFSET});

AN725

26 Rev. 0.1

For example, to locate the stack at address 0x2000_6000:

PROVIDE(_vStackTop = 0x20006000);

Instead of editing the link scripts file, the stack offset change can be made inside the IDE:

1. Right-click on the project_name in the Project Explorer view.

2. Select Properties.

3. Click on C/C++ BuildSettingsTool Settings tabMCU LinkerTarget and input the desired stack offset into the Stack offset field. This offset will occur from the top address of RAM, and the value must be a multiple of 4. Figure 17 shows this dialog in the Precision32 IDE.

Figure 17. Using the Precision32 IDE to Select the Project Library

After changing either the custom linker script or IDE settings, clean and rebuild the project. View the map file for theproject to verify the stack location changed as desired.

AN725

Rev. 0.1 27

6. General Power-Saving Tips

6.1. PinsPlacing any unused pins in analog mode using the PBSTD module (PBMDSEL.x = 0) disables the drivers andweak pull-ups on the pin. The pin will float and consume little power.

For pins that are connected to external hardware, place the pins in analog mode if possible. If this is not possible,place the pins in a natural state that will consume no power. For example, if a pin has an external pull-downresistor, putting a ‘0’ in the latch will draw no current from the external pull-down.

If the system configuration allows it, disabling weak pull-ups on a port-by-port basis may also help reduceunnecessary power consumption in the pins.

6.2. PeripheralsBefore entering the low power mode, disable any unwanted peripherals. Stopping the clock to the peripheral maydisable the peripheral, but if the peripherals operate on a clock independent from the AHB or APB clocks, only theperipheral registers will be disabled. Disabling the module explicitly (DCDCEN = 0 for the DCDC0 module, forexample) will ensure the module does not draw extra power in the low power mode.

6.3. BiasesIn addition to peripherals, disable any unneeded bias sources in the device before entering the low power mode.These biases draw current to provide voltage references inside the chip. One example of a bias is the 1.2/2.4 VVREF0 module or the ADC internal 1.65 V voltage reference.

6.4. ClocksExternal oscillators require a bias to start and maintain oscillation. If possible, switching to the internal oscillatorsand stopping the external oscillator will reduce the power consumption. The internal precision oscillator alsorequires extra current to operate. In most cases, switching the AHB and APB clock to the Low Power Oscillator(LPOSC0) or RTC0 oscillator before entering the low power mode will result in the lowest power consumption.

The SiM3L1xx devices feature programmable clocks for each peripheral. When entering a low power mode,disabling the clocks to any unused peripherals will reduce the system power consumption.

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DisclaimerSilicon Labs intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Labs products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. Silicon Labs reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. Silicon Labs shall have no liability for the consequences of use of the information supplied herein. This document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. The products are not designed or authorized to be used within any Life Support System without the specific written consent of Silicon Labs. A "Life Support System" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. Silicon Labs products are not designed or authorized for military applications. Silicon Labs products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological or chemical weapons, or missiles capable of delivering such weapons.

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